(a) Field of the Invention
The present invention relates to a light emitting display and a driving method thereof. More specifically, the present invention relates to an organic electroluminescent (EL) display.
(b) Description of the Related Art
In general, an organic EL display electrically excites a phosphorous organic compound to emit light, and it voltage- or current-drives N×M organic emitting cells to display images. As shown in FIG. 1, the organic emitting cell includes an anode (e.g., indium tin oxide (ITO)), an organic thin film, and a cathode layer (metal). The organic thin film has a multi-layer structure including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) for maintaining balance between electrons and holes and improving emitting efficiencies. Further, the organic emitting cell includes an electron injecting layer (EIL) and a hole injecting layer (HIL).
Methods for driving the organic emitting cells include a passive matrix method, and an active matrix method using thin film transistors (TFTs) or metal-oxide semiconductor field-effect transistors (MOSFETs). In the passive matrix method, cathodes and anodes are arranged to cross (i.e., cross over or intersect) with each other, and lines are selectively driven. In the active matrix method, a TFT and a capacitor are coupled to each ITO pixel electrode to thereby maintain a predetermined voltage according to capacitance of the capacitor. The active matrix method is classified as a voltage programming method or a current programming method according to signal forms supplied for programming a voltage in the capacitor.
FIG. 2 shows a conventional pixel circuit of a voltage programming method for driving an organic EL element (OLED), and FIG. 3 shows a driving waveform diagram for driving the pixel circuit shown in FIG. 2.
As shown in FIG. 2, the conventional pixel circuit following the voltage programming method includes transistors M1, M2, M3, and M4, capacitors C1 and C2, and an OLED.
The transistor M1 controls the current flowing to a drain according to a voltage applied between a gate and a source, and the transistor M2 programs a data voltage to the capacitor C1 in response to a select signal from a scan line Sn. The transistor M3 diode-connects the transistor M1 in response to a select signal from a scan line AZn. The transistor M4 transmits the current of the transistor M1 to the OLED in response to a select signal from a scan line AZBn.
The capacitor C1 is coupled between the gate of the transistor M1 and a drain of the transistor M2, and the capacitor C2 is coupled between the gate and the source of the transistor M1.
An operation of the conventional pixel circuit will be described with reference to FIG. 3.
When the transistor M3 is turned on by the select signal from the scan line AZn, the transistor M1 is diode-connected, and a threshold voltage of the transistor M1 is stored in the capacitor C2.
When the transistor M3 is turned off and a data voltage is applied, a voltage that corresponds to a summation of a variation of the data voltage applied to the data line Dm and the threshold voltage of the driving transistor M1 is stored in the capacitor C2 because of a boosting operation by the capacitor C1. When the transistor M4 is turned on, a current corresponding to the data voltage flows to the OLED.
The conventional pixel circuit uses two capacitors C1 and C2 and transistors M3 and M4 to compensate for deviations of the threshold voltage of the transistor M1, but the pixel circuit and a driving circuit become complicated and an aperture ratio of the light emitting display is reduced since the conventional pixel circuit requires three different scan lines. Also, since the data is programmed after the deviation of the threshold voltage is compensated during a single pixel selecting time, it is difficult to apply the pixel circuit to a high-resolution panel because of a data charging problem.